Traditionally, a lead-acid storage battery has been used for a vehicle battery or an industrial battery against an instantaneous voltage drop when a power failure occurs at an office building, a hospital, or the like. Recently, a vehicle that has adopted the ISS system has been marketed in the automobile industry. In the ISS system, while the vehicle stops due to a traffic signal or the like, the engine of the vehicle is temporarily stopped, and the lead-acid storage battery is used as a power source during the stop of the vehicle or a power source for restarting the engine when the vehicle starts moving. It has also been studied that electric power generated by the use of natural energy obtained by the solar power generation, the wind power generation, or the like is regulated by the lead-acid storage battery, or energy obtained by the power generation is temporarily stored in the lead-acid storage battery to use as power for an electric vehicle or the like.
In a system using the lead-acid storage battery for the above-mentioned ISS system or the above-mentioned applications of the solar power generation and the wind power generation, electric power is generated by a power generator (such as an engine power generator for a vehicle, a solar battery, or a wind power generator), and the electric power obtained by the power generation is converted to direct current by a converter to charge the lead-acid storage battery. On the contrary, when discharging is performed from the lead-acid storage battery, electric power is converted to alternating current by an inverter, and is then supplied to a load.
The lead-acid storage battery to be used in this manner has a characteristic that, when the lead-acid storage battery in a fully-charged state is to be further charged, water in the electrolyte of the lead-acid storage battery is decomposed by a charging current. Thus, it is a practice to use the lead-acid storage battery in a charged state (that is commonly referred to as a State Of Charge. Hereinafter referred to as “SOC”) set to a lower state (e.g., SOC: 60%) than the fully-charged state so as not to waste generated electric power. When the lead-acid storage battery is used for a long time by this usage, crystals of lead sulfate generated in a negative-electrode active material inside the lead-acid storage battery coarsen. Thus, the crystals are not dissolved even if the lead-acid storage battery is charged. Then, these crystals of lead sulfate are accumulated to bring about deterioration of the lead-acid storage battery. As a result, the life of the lead-acid storage battery is shortened. Further, a plurality of the lead-acid storage batteries are connected in series for use. Consequently, a variation occurs in the amount of the crystals of lead sulfate that are not dissolved, for each battery.
So-called “refresh charging” (that may also be referred to as “equalizing charging”) is performed in order to solve this problem (refer to JP 2010-020906 A [Patent Document 1], for example). In this refresh charging, each of the lead-acid storage batteries connected in series is charged to a completely-charged state (SOC: 100%, or the fully-charged state) for each given period to remove lead sulfate accumulated in the negative-electrode active material.
The charging method described in Patent Document 1 is characterized in that, after each lead-acid storage battery has been charged with a constant current until the voltage of the lead-acid storage reaches a set voltage (E), charging with the set voltage (E) is continued until a charging current reaches a set current value or less.